Double jet process for producing nanosilver dispersions

ABSTRACT

This invention provides a double jet process to produce silver nanoparticles, the process comprising providing a double-jet system to form a reaction mixture in a reactor containing the reactor solution by adding a basic aqueous silver ammonia complex solution and a basic reducing solution to the reactor solution at the same controlled rate with a targeted pH profile for the reactor solution from acidic to basic determined by the addition rate and the initial pH of the reactor solution. Use of this process results in a dispersion comprised of silver nanoparticles that have a specific size and de-agglomeration level that is determined by the process conditions.

FIELD OF THE INVENTION

The invention is directed to a double jet process for producing silver powders comprising silver nanoparticles with spherical morphology. The silver powders produced are particularly useful in electronic applications.

BACKGROUND OF THE INVENTION

Silver powder is used in the electronics industry for the manufacture of conductor thick film pastes. The thick film pastes are screen printed onto substrates forming conductive circuit patterns. These circuits are then dried and fired to volatilize the liquid organic vehicle and sinter the silver particles. Printed circuit technology is requiring denser and more precise electronic circuits. To meet these requirements, the conductive lines have become narrower in width with smaller distances between lines. Several printing technologies have been developed to achieve these requirements, e.g., ink-jets and other jetting technologies. The silver powder particles necessary for these new printing technologies are nanoparticles (less than 100 nm), uniform in size, and well dispersed.

Many processes currently used to manufacture metal powders can be applied to the production of silver nanoparticles. Silver nanoparticles used in electronic applications are generally manufactured using chemical precipitation processes. Silver nanoparticles are produced by chemical reduction in which an aqueous solution of a soluble salt of silver is reacted with an appropriate reducing agent under conditions such that silver powder is precipitated.

There is a need for a process to efficiently produce well dispersed silver nanoparticles particles, with capability to control the particle size.

SUMMARY OF THE INVENTION

This invention provides a process for preparing a silver dispersion comprising silver nanoparticles, wherein the silver nanoparticles have a specific size that is determined by the process conditions and the use of one or more dispersing agents in the process.

One embodiment provides a process for preparing silver nanoparticles, said process comprising the steps of:

(a) preparing a basic aqueous silver ammonia complex solution comprising a water soluble silver salt and ammonia in deionized water;

(b) preparing a basic reducing solution comprising hydrazine monohydrate in deionized water;

(c) preparing an acidic reactor solution with a pH in the range of 3.0 to 5.0, said reactor solution comprising:

-   -   (i) one or more dispersing agents, one of which is gum arabic         hydrated in deionized water; and     -   (ii) nitric acid;

(d) maintaining the basic aqueous silver ammonia complex solution and the basic reducing solution at the same temperature T_(M), wherein T_(M) is in the range of 10° C. to 90° C.;

(e) providing a double-jet system to form a reaction mixture in a reactor containing the reactor solution by adding the basic aqueous silver ammonia complex solution and the basic reducing solution to the reactor solution at the same controlled rate with a targeted pH profile for the reactor solution from acidic to basic determined by the addition rate and the initial pH of the reactor solution; and

(f) maintaining the reaction mixture in the reactor at constant temperature in the range of 10° C. to 90° C. to produce the silver nanoparticles.

In another embodiment the process further comprises the steps of:

(g) separating the silver nanoparticles from the reaction mixture;

(h) washing the silver nanoparticles with deionized water; and

(i) transferring the silver nanoparticles to a nonaqueous solution

DETAILED DESCRIPTION OF INVENTION

This invention provides a double jet process to efficiently produce silver nanoparticles. Use of this process results in a dispersion comprised of silver nanoparticles that have a specific size and de-agglomeration level that is determined by the process conditions and the use of one or more particle dispersing agents. These silver particles are highly uniform and highly dispersible.

The basic silver-ammonia complex aqueous solution is prepared by adding a water soluble silver salt to deionized water. In one embodiment the soluble salt is silver nitrate. Ammonia is added to make the silver-ammonia complex. The molar ratio of ammonia to silver is greater than 2. Small amounts of nitric acid (1%-5%) can be added to this solution to increase the stability of the silver complex.

The basic reducing solution is also an aqueous solution and is prepared by adding hydrazine monohydrate to deionized water.

The acidic reactor solution is prepared by dissolving gum arabic in deionized water and letting the gum arabic hydrate in the water for a period of one to 24 hours. The gum arabic acts as a dispersing agent. Nitric acid and one or more additional dispersing agents are then added to the mixture. The nitric acid serves to control the initial pH of the solution. The dispersing agent serves to prevent aggregation and coat the particle.

The process is typically run such that the initial pH of the reactor solution and the addition rate of the basic solutions are utilized to control the size of the silver nanoparticles. This pH is adjusted by adding sufficient nitric acid to the reactor solution. The pH of the reactor solution is in the range of 3.0 to 5.0, preferably 3.5 to 4.5.

In addition, an additional dispersing agent selected from benzotriazole, salts of polynaphthalene sulfonate formaldehyde condensate such as Daxad™ 19 (manufactured by Hampshire Chemical Corp., division of Dow Chemical Co.), Pluronic™, poloxamer block copolymer (manufactured by BASF Corp.), ethoxylated phenols such as Gafac™, (manufactured by GAF Corp.), polyethylene glycol with molecular weight ranges from 200 to 8000, and mixtures of these surfactants can be added to the reactor solution. The amount of this surface modifying dispersing agent ranges from 0.001 g per gram of silver to greater than 0.2 grams per gram of silver. The preferred range to make finely divided particles is from 0.04 to 0.20 grams per gram of silver.

In typical processes the silver precipitation is carried at high pH. The instant process starts at low pH by using a double jet process. In this process the basic reducing solution and the basic silver ammonia complex are outside the reactor initially and then added slowly into the reactor. The initial pH of the reactor is in the range of 3.5 to 4.5. At this starting pH range, hydrazine hydrate is not capable of reducing silver ions at an observable rate. During the addition of the two basic solutions, the pH is increased and the reaction does proceed when the pH is raised to greater than about 8.0. By adjusting the starting pH, the concentration of silver ions present in the system when the threshold pH needed for reduction is attained can be varied, and thus the size of the particles controlled.

When an additional surface modifying dispersing agent is added to the precipitations, the surface of the resulting silver is further modified as the surface. By changing the type of additional dispersing agent and its functional groups the cleanliness of the surface of the particles and the sintering behavior are modified.

The particle size d₅₀ for the silver nanoparticles is from 10 to 90 nm. The SEM size of the particles can also be determined directly from field emission scanning electron microscope (FESEM) images. The ratio of d₅₀ to SEM size is in the range of 1-2.

In one process embodiment the silver particles are spherical. In one such embodiment the combination of dispersing agents is the gum arabic and polyethylene glycol.

EXAMPLES

The following examples and discussion are offered to further illustrate, but not limit the process of this invention. Note that particle size distribution numbers (d₁₀, d₅₀, d₉₀) were measured using a LM-10 Particle Size Analyzer from Nanosight UK. The d₁₀, d₅₀ and d₉₀ represent the 10th percentile, the median or 50th percentile and the 90th percentile of the particle size distribution, respectively, as measured by following the Brownian motion of the nanoparticles. That is, the d₅₀ d₉₀) is a value on the distribution such that 50% (10%, 90%) of the particles have a volume of this value or less. The particle size d₅₀ for the silver nanoparticles is from 10 to 90 nm. An average particle size, the SEM size of the particles, can be determined directly from field emission scanning electron microscope (FESEM) images.

Example 1

The basic silver-ammonia complex solution was prepared by dissolving 63 g of silver nitrate in 200 g of deionized water. Then 75 ml of a 30% ammonia solution was added to form the complex (ammonia to silver molar ratio must be larger than 2). This solution was kept at 25° C. while continuously stirring. The basic reducing solution was prepared by adding and dissolving 9.5 ml of hydrazine monohydrate to 200 nil of deionized water in a separate container from the silver-ammonia complex solution. This solution was kept at 25° C. while continuously stirring. The reactor solution was prepared by adding and dissolving 12 g of arabic gum to 1600 g of deionized water. This solution was kept at 25° C. while continuously stirring for 16 hours. Then 2.5 ml of nitric acid solution (1 N) was added followed by the addition of 0.5 g of polyethylene glycol (mw 8000).

After the three solutions were prepared, the silver-ammonia complex solution and the reducing solution were added to the reactor solution at equal addition rates for a total addition time of 30 minute to make the reaction mixture. The reaction mixture is continuously stirred during the addition. After the addition was completed, the reaction mixture was stirred for 30 minutes. To separate the nanoparticles, the gum arabic in the reaction mixture was hydrolyzed by adjusting the pH of the reaction mixture to 12.5 using sodium hydroxide and the temperature was increased to 85° C. for 4 hours. The reaction mixture was decanted and the silver nanoparticle slurry was washed using decantation followed by dialysis until a conductivity of the wash water was less than or equal to 8 microsiemans.

The nanoparticle silver dispersion consisted of silver particles with an average size of 25 nm as obtained from the field emission scanning electron microscope image. Sizes were also measured by Brownian motion. The d₁₀, d₅₀, and d₉₀ were 12 nm, 21 nm and 40 nm, respectively.

Example 2

The conditions used were essentially the same as used in Example 1 with the exception that the polyethylene glycol was not added to the reactor solution.

The nanoparticle silver dispersion consisted of silver particles with an average size of 24 nm as obtained from the field emission scanning electron microscope image. Sizes were also measured by Brownian motion. The d₁₀, d₅₀, and d₉₀ were 13 nm, 22 nm and 52 nm, respectively.

Example 3

The conditions used were essentially the same as used in Example 1 with the exception that the reactor solution contains 5 ml (1N) of nitric acid.

The nanoparticle silver dispersion consisted of silver particles with an average size of 18 nm as obtained from the field emission scanning electron microscope image.

Comparative Experiment A

The conditions used were essentially the same as used in Example 1 with the exception that the pH of the reactor solution is adjusted to 10 by adding hydrazine hydrate as base. The reduction of silver complex in this case is almost instantaneous with the precipitation of the metal occurring in the first 1-2 seconds. The resulting particles are spherical, polycrystalline and have an average diameter of about 350 nm as obtained from the field emission scanning electron microscope image. 

What is claimed is:
 1. A process for preparing silver nanoparticles, said process comprising the steps of: (a) preparing a basic aqueous silver ammonia complex solution comprising a water soluble silver salt and ammonia in deionized water; (b) preparing a basic reducing solution comprising hydrazine monohydrate in deionized water; (c) preparing an acidic reactor solution with a pH in the range of 3.0 to 5.0, said reactor solution comprising: (i) one or more dispersing agents, one of which is gum arabic hydrated in deionized water; and (ii) nitric acid; (d) maintaining the basic aqueous silver ammonia complex solution and the basic reducing solution at the same temperature T_(M), wherein T_(M) is in the range of 10° C. to 90° C.; (e) providing a double-jet system to form a reaction mixture in a reactor containing the reactor solution by adding the basic aqueous silver ammonia complex solution and the basic reducing solution to the reactor solution at the same controlled rate with a targeted pH profile for the reactor solution from acidic to basic determined by the addition rate and the initial pH of the reactor solution; and (f) maintaining the reaction mixture in the reactor at constant temperature in the range of 10° C. to 90° C. to produce the silver nanoparticles.
 2. The process of claim 1 further comprising the steps of: (h) separating the silver nanoparticles from the reaction mixture; (i) washing the silver nanoparticles with deionized water; and (j) transferring the silver nanoparticles to a nonaqueous solution
 3. The process of claim 1, wherein said water soluble silver salt is silver nitrate.
 4. The process of claim 1, wherein said one or more additional dispersing agents are selected from the group consisting of benzotriazole, salts of polynaphthalene sulfonate formaldehyde condensate, poloxamer block copolymer, ethoxylated phenols, polyethylene glycol with molecular weight ranges from 200 to 8000, and mixtures thereof.
 5. The process of claim 1, wherein the molar ratio of ammonia to silver is greater than
 2. 